1. Field of the Invention:
The present invention relates to a heat pump cycle system in which pressure of refrigerant discharged from a compressor exceeds the critical pressure and carbon dioxide (CO.sub.2) is used as refrigerant. The heat pump cycle system can set cooling operation and heating operation.
2. Description of Related Art:
A vapor-compression type refrigerant cycle using carbon dioxide (CO.sub.2) as refrigerant (hereinafter, referred to as "CO.sub.2 refrigerant cycle") is disclosed in JP-A-9-264622 by the applicant of the present invention. To increase cooling capacity of the CO.sub.2 refrigerant cycle, pressure of high-pressure side refrigerant is need to be increased. However, when the pressure of high-pressure side refrigerant is simply increased, coefficient of performance of the CO.sub.2 refrigerant cycle is deteriorated. Thus, in the conventional CO.sub.2 refrigerant cycle, temperature of refrigerant on an outlet side of a radiator is controlled, so that cooling capacity of the CO.sub.2 refrigerant cycle is controlled while the coefficient of performance of the CO.sub.2 refrigerant cycle becomes higher. However, the conventional CO.sub.2 refrigerant cycle is only for increasing the cooling capacity of the CO.sub.2 refrigerant cycle. Therefore, when the CO.sub.2 refrigerant cycle is applied to a heat pump cycle system which can selectively switch cooling operation and heating operation, the pressure of high-pressure side refrigerant where the coefficient of performance becomes maximum during the cooling operation does not correspond to the pressure of high-pressure side refrigerant where the coefficient of performance becomes maximum during the heating operation. As a result, when the control of the cooling operation is simply applied to the heating operation in the conventional CO.sub.2 refrigerant cycle, the coefficient of performance during the heating operation is deteriorated.
Further, in the CO.sub.2 refrigerant cycle, CO.sub.2 refrigerant is in a supercritical state on the high-pressure side and CO.sub.2 refrigerant is not condensed in a radiator. Therefore, as CO.sub.2 refrigerant flows from a refrigerant inlet side toward a refrigerant outlet side within the radiator, temperature of CO.sub.2 refrigerant is gradually decreased. Thus, temperature of air blown into a passenger compartment is approximately the average temperature of radiator. To increase the temperature of air blown into the passenger compartment during the heating operation, the temperature of CO.sub.2 refrigerant on the refrigerant inlet side of the radiator is need to be increased so that the average temperature of air passing through the radiator is increased. In this case, because it is necessary to increase the pressure of CO.sub.2 refrigerant discharged from the compressor, the coefficient of performance of the CO.sub.2 refrigerant cycle may be deteriorated.